The present invention is directed to an integral delivery valve/regulator for pressurized gas storage containers. In particular, the present invention is directed to an integral delivery valve/regulator for pressurized gas storage containers that requires sub-atmospheric pressure to enable the withdrawal of the gas from the container.
Toxic and other hazardous specialty gases are used in a number of industrial applications, including semiconductor device fabrication. Many users of these hazardous specialty gases are concerned about the possibility of an unintentional release. By virtue of having a positive gauge pressure, pressurized gases in cylinders will be released immediately once a shut-off valve attached to the pressurized cylinder is opened. Even with a gas-tight outlet cap in place (as required for most hazardous gases), unintentional opening of the valve can lead to serious consequences when the cap is removed. Although always undesirable, a hazardous gas release may be particularly undesirable in semiconductor processing applications. Such a release would necessitate a partial or complete evacuation of the semiconductor processing factory, leading to substantial losses in scrap product and unscheduled downtime. Also, the sensitive and expensive equipment used in semiconductor processing factories may be damaged by exposure to even traces of the hazardous gas.
Many hazardous gas containers are outfitted with restrictive flow orifices in the valve outlet to limit the rate of release of the gas in the event of an accidental release. Although a restrictive flow orifice may significantly reduce the hazardous gas release rate, any release can still cause a considerable disruption to operations, and the hazard risk to personnel will not be totally eliminated. Furthermore, flow restriction may be unacceptable due to impracticably limiting the flow of the gas while the cylinder is in service. Excess flow sensors coupled to automatic shut-off valves can shut off flow in the event of a leak in a delivery system, but will only be effective when the release is substantially larger than the delivery flow-rate and if it occurs downstream of the automatic shutoff valve. It may also be possible to trigger a shut-off valve based on a hazardous gas monitor near the possible leakage points. All such systems, however, are complex and costly, and are only effective for gas containers that are already properly installed in a gas delivery system. Many semiconductor manufacturing processes, such as ion-implantation, chemical vapor deposition, reactive ion etching, high-density plasma etching, and the like, use hazardous gases at sub-atmospheric pressure (i.e. below ambient pressure). As a result, the gas cylinder need not provide the gas with a positive gauge pressure in all cases.
For the purposes of the present invention, the term gas, as indicated herein, encompasses both a permanent gas and a vapor of a liquified gas. Permanent gases are gases which, practically, cannot be liquified by pressure alone. Vapors of liquified gases are present above the liquid in a compressed gas cylinder. Gases which liquify under pressure as they are compressed for filling into a cylinder are not permanent gases and are more accurately described as liquified gases under pressure or as vapors of liquified gases.
One approach to providing sub-atmospheric gas delivery is a method described by Knollmueller in U.S. Pat. No. 4,744,221 and by Tom, et al. in U.S. Pat. Nos. 5,518,528, 5,704,965 and 5,704,967 wherein a hazardous gas is physically or chemically adsorbed on the surface of a sorbent within a container to lower the equilibrium pressure of the desired species in the container. While this method has been employed for the storage and delivery of certain gases (see, e.g., McManus, J. V. et al., Semiconductor Fabtech, Volume 7, 1998), the method has significant limitations. First, the amount of gas stored in a given volume of the adsorbent used is relatively small compared to a liquefied compressed gas (e.g. phosphine) thereby requiring a relatively large vessel which utilizes valuable footprint space, which is important, for example, when these gases are used in a semiconductor fabrication cleanroom. Also, heat transfer limitations in the solid sorbent will limit the rate at which gas can be desorbed compared to that from a compressed gas (e.g. silicon tetrafluoride).
Knollmueller (U.S. Pat. No. 4,744,221) describes a process of adsorbing a gas onto a solid sorbent so that the equilibrium pressure of the gas is reduced inside of a vessel. By heating the vessel, the equilibrium pressure in the vessel could be increased and permit the delivery of the gas at above-atmospheric pressure. However, heating of specialty gases is undesirable because it may be slow, hard to control and cause decomposition of the gas. Also, when heated so that the delivery pressure is increased, there is decreased protection against accidental release of the gas.
Tom, et al. (U.S. Pat. No. 5,518,528 and subsequently U.S. Pat. Nos. 5,704,965 and 5,704,967) improved on this concept by using a sorbent where the gas could be released without substantial decomposition by reducing the downstream pressure. These sorbents still have a disadvantage of needing to be optimized for each sorbate (hazardous gas). Further, the equilibrium pressure in the vessel in this system is constantly being decreased as product is withdrawn. This phenomenon makes gas flow control more difficult and limits the fraction of the gas charged into the vessel that may be withdrawn by the user. Also, in the event that the ambient temperature increases, the pressure inside the vessel could potentially increase above atmospheric pressure, decreasing the protection against accidental release. Conversely, at cooler temperatures, there may not be sufficient pressure to deliver the gas.
An additional concern when storing hazardous gases under sub-ambient pressures is the likelihood of inboard contamination of the vessel in the event of a leak due to the vacuum. Not only will this atmospheric contamination adversely affect the purity the gas, but, with respect to the above method, it could also conceivably react with the adsorbed gas stored under sub-ambient pressure and generate heat, pressure or corrosive by-products. An additional problem with this method is that the pressure of the gas being delivered is a function of both the quantity of adsorbed gas remaining and the temperature of the adsorbent. Hence, the pressure in the vessel containing the adsorbed gas could easily exceed atmospheric pressure if the contents are heated. Also, the delivery pressure undesirably decreases as the contents of the vessel are depleted. Eventually, the delivery pressure diminishes to a point where sufficient flow can no longer be sustained. At this point, the source must be replaced, even though there may be substantial inventory of gas remaining in the adsorbed phase relative to the initial charge.
Another approach to providing sub-atmospheric gas delivery is a device described by Le Febre et al. in U.S. Pat. No. 5,937,895. Here, the device provides a regulator that uses a valve element that is responsive, in one embodiment, to a vacuum condition downstream of the regulator. The valve only allows flow when this vacuum condition occurs downstream of the valve such that the possibility of accidental spillage or release of toxic liquid or gases is reduced. Note that no sorbents are used as described in the Knollmueller and Tom patents. This patent also teaches use of its sub-atmospheric gas delivery device with an internal flow restriction within the storage container as disclosed in U.S. Pat. No. 6,045,115. This flow restrictor provides a capillary size opening that limits the discharge of gas phase fluid from the pressurized container. As indicated in the '115 patent, liquid discharge from the container may be particularly hazardous since the mass rate of discharge of liquid will greatly exceed the mass rate of discharge of the corresponding gas through a particular opening. The '115 patent locates the entry point of the capillary flow restrictor at approximately the midpoint of the length of the cylinder. This therefore prevents discharge of a liquid in the cylinder whether the cylinder is upside down or right side up. However, a negative aspect of this design is that the capillary system may be prone to clogging. Once plugged, the cylinder would be difficult or impossible to empty of the hazardous gas.
To accomplish the same result of preventing discharge of a liquid, PCT Patent Application No. PCT/US99/09137, teaches use of a pressurized container which uses a phase separation device, which is a porous membrane that is permeable to vapor or gas deriving from liquid in the container, but is not permeable to the liquid. Here, the phase separator is disposed upstream of the pressure regulator so that fluid is prevented from entering and interfering with the function of the regulator and preventing egress of liquid from the vessel. The regulator is a flow device which can be set at a predetermined level to dispense gas or vapor from the container at a vessel pressure level which may be superatmospheric, sub-atmospheric, or atmospheric pressure, depending on dispensing conditions.
The present invention overcomes the limitations of the prior art by reducing the pressure to sub-atmospheric mechanically, rather than by sorption and by use of a high pressure valve upstream of the regulator. While negative pressure regulators (also known as absolute pressure regulators or vacuum regulators) are well-known, by placing this functionality integral to a gas storage and delivery package, use of one provides the unique benefits not afforded by a stand-alone regulator. This integral valve/regulator, which may be pre-set and locked to provide only sub-ambient pressures, beneficially reduces the risk of accidental release of gases.
European Patent Application EP 0 916 891 A2 discloses a modular gas control valve having a high pressure shut off valve upstream of a regulator. Here, the purpose of the shut-off valve is for dispensing control. The system taught here is for a standard compressed gas system, not for a system that only provides the gas when the pressure downstream of the regulator is sub-atmospheric. The use in preventing liquid interfering with the regulator is not taught.
None of the prior art teaches an apparatus for containing and delivering hazardous gases at sub-atmospheric pressure from a pressurized container which includes a sub-atmospheric pressure regulator to allow the gas in the container to be delivered only when the pressure sensing means senses a downstream pressure at or below a pre-set pressure, and which includes a high pressure shut-off valve upstream of the pressure regulator. The gas may flow only when said outlet orifice of the apparatus is connected to a vacuum system. The high pressure valve upstream of the regulator in the integral valve/regulator provides numerous advantages as will be discussed in detail below.
It is principally desired to provide an apparatus for containing and delivering hazardous gases from a pressurized container.
It is further desired to provide an apparatus for containing and delivering hazardous gases at sub-atmospheric pressure from a pressurized container.
It is still further desired to provide an apparatus for containing and delivering hazardous gases that reduces the possibility of accidental spills or release of the hazardous gas.
It is also further desired to provide an apparatus for containing and delivering hazardous gases without the need for sorbents to control the handling, storage, and delivery of toxic fluids.
It is further desired to provide an apparatus for containing and delivering hazardous gases that may only discharge its contents when placed in service with a vacuum system.
It is still further desired to provide an apparatus for containing and delivering hazardous gases that can only dispense a hazardous gas when conditions downstream of the apparatus are at a desired pressure less than atmospheric pressure.
It is also further desired to provide an apparatus for containing and delivering hazardous gases that does not require use of a restrictive flow orifice.
It is further desired to provide an apparatus for containing and delivering hazardous gases that does not require use of excess flow sensors coupled to automatic shut-off valves to shut off unintentional flow of the hazardous gas.
It is still further desired to provide an apparatus for containing and delivering hazardous gases without the need for a shut-off valve coupled to a hazardous gas monitor near possible leakage points.
It is also further desired to provide an apparatus for containing and delivering hazardous gases at sub-atmospheric pressure where the likelihood of inboard contamination of the vessel in the event of a leak due to the vacuum is minimized.
Finally, it is desired to provide an apparatus for containing and delivering hazardous gases which requires a relatively small amount of space.